We want to pursue a novel line of research regarding vestibular influences upon arm reaching movements. The proposed experiments have been motivated by recent findings in human subjects showing that path curvature is greater, acceleration duration is shorter and peak acceleration is increased for upward versus downward arm movements. Muscle EMG activity and computational modeling have suggested that these kinematic asymmetries are generated because the brain uses gravity as a major force for acceleration during downward movements and for deceleration during upward movements. Here, we propose a basic set of experiments to examine whether vestibular (otolith) signals are functionally important for these properties. Thus, the aims of this application are two-fold. First, to ascertain whether macaques exhibit similar kinematic and EMG asymmetries during vertical arm movements as those observed in humans. Second, to examine whether the observed up/down asymmetries in arm movement kinematics and EMG depend on a functional vestibular labyrinth. We hypothesize (1) that macaques, like humans, show kinematic and EMG asymmetries during execution of up and down (as compared to horizontal) arm movements and (2) that an intact vestibular labyrinth is functionally important for detecting gravity and allowing for cost-effective planning of vertical arm movements. To achieve these goals, we will measure arm movement kinematics (Optotrak), as well as EMG activity from agonists and antagonists as monkeys execute center-out and center-in arm movements both in normal animals and after bilateral labyrinthectomy. To further examine whether there is any long term adaptation, we will be testing kinematics and EMG weekly for the first two months after labyrinthectomy. These experiments will be the first to directly investigate in macaques a potential role of gravity-related otolith signals on the planning and execution of arm movements. If successful, we will establish a solid behavioral context for a vestibular influence upon arm movements. Future neurophysiology studies can then explore how gravity- related vestibular signals are used for arm movement planning and execution.